1. Field of the Invention
This invention relates to a device and method for separating heavier and lighter if, fractions of a fluid sample. More particularly; this invention relates to a device and method for collecting and transporting fluid samples whereby the device and fluid sample are subjected to centrifugation in order to cause separation of the heavier fraction from in the lighter fraction of the fluid sample.
2. Description of Related Art
Diagnostic tests may require separation of a patient""s whole blood sample into components, such as serum or plasma, the lighter phase component, and red blood cells, the heavier phase component. Samples of whole blood are typically collected by venipuncture through a cannula or needle attached to a syringe or an evacuated collection tube. Separation of the blood into serum or plasma and red blood cells is then accomplished by rotation of the syringe or tube in a centrifuge. Such arrangements use a barrier for moving into an area adjacent the two phases of the sample being separated to maintain the components separated for subsequent examination of the individual components.
A variety of devices have been used in collection devices to divide the area between the heavier and lighter phases of a fluid sample.
The most widely used device includes thixotropic gel materials such as polyester gels in a tube. The present polyester gel serum separation tubes require special manufacturing equipment to prepare the gel and to fill the tubes. Moreover, the shelf-life of the product is limited in that overtime globules may be released from the gel mass. These globules have a specific gravity that is less than the separated serum and may float in the serum and may clog the measuring instruments, such as the instrument probes used during the clinical examination of the sample collected in the tube. Such clogging can lead to considerable downtime for the instrument to remove the clog.
No commercially available gel is completely chemically inert to all analytes. If certain drugs are present in the blood sample when it is taken, there can be an adverse chemical reaction with the gel interface.
Therefore, a need exists for a separator device that (i) is easily used to separate a blood sample; (ii) is independent of temperature during storage and shipping; (iii) is ti stable to radiation sterilization; (iv) employs the benefits of a thixotropic gel barrier yet avoids the many disadvantages of placing a gel in contact with the separated blood components; (v) minimizes cross contamination of the heavier and lighter phases of the sample during centrifugation; (vi) minimizes adhesion of the lower and higher density materials against the separator device; (vii) is able to move into position to form a barrier in less time than conventional methods and devices; (viii) is able to provide a clearer specimen with less cell contamination than conventional methods and devices; and (ix) can be used with standard sampling equipment.
The present invention is a method and assembly for separating a fluid sample into a higher specific gravity phase and a lower specific gravity phase. Desirably, the assembly of the present invention comprises a plurality of constituents. Preferably, the assembly comprises a container and a composite element.
Most preferably, the container is a tube and the composite element is a separator arranged to move in the tube under the action of centrifugal force in order to separate the portions of a fluid sample.
Most preferably, the tube comprises an open end, a closed end and a sidewall extending between the open end and closed end. The sidewall comprises an outer surface and an inner surface. The tube further comprises a closure disposed to fit in the open end of the tube with a resealable septum. Preferably, the separator element is releaseably positioned at the open end of the tube with the closure. Alternatively, the separator element may also be releasably positioned at the closed end of the tube.
Alternatively, both ends of the tube may be open, and both ends of the tube may be sealed by elastomeric closures. At least one of the closures of the tube may include a needle pierceable resealable septum.
Preferably, the separator is sealingly engaged with portions of the tube near the open top. The separator may be formed from a needle-pierceable resealable material that enables a needle cannula to be passed therethrough for depositing a specimen into the tube. The separator may be formed from a material that exhibits good sealing characteristics against the inner surface of the cylindrical sidewall of the tube, and may be diametrically dimensioned for sealing engagement against the sidewall of the tube. Thus the separator will isolate material on one side of the separator from material on the opposed side of the separator.
The separator may comprise a resiliently deformable material, such as thermoplastic elastomeric foam. The elastomeric portions of the separator are readily deformable and provide desirable sealing characteristics against the sidewall of the tube.
The separator further comprises a higher density portion integrally engaged with or embedded in the less dense elastomer. The more dense material preferably is disposed at a lower end of the separator. Thus, the higher density material functions to deform the separator downwardly into a smaller cross-sectional dimension during centrifugation. The more dense material also functions to define an overall specific gravity or density for the separator that lies between the specific densities of the different phases of blood or other such liquid to be separated. The elastomeric portions of the separator may be at least partly hollowed to facilitate the deformation during centrifugation and to facilitate the needle piercing.
In use, a fluid enters the assembly by needle. The needle penetrates the closure and through the foam or other elastomeric portions of the separator. The needle is withdrawn from the assembly and the septum of the closure and the separator reseals. The assembly then is placed in a centrifuge, and a centrifugal load is applied. The centrifugal load causes the more dense material embedded at the lower end of the separator to move downwardly in the tube, thereby elongating the separator and reducing the cross-sectional dimensions of the separator. As a result, the separator is able to move freely within the tube and moves into contact with the fluid to be separated. Simultaneously, the more dense phase of the fluid will move toward the lower end of the tube, while the less dense phase of the fluid will flow around the separator. The fluid eventually will be substantially divided into two separate phases with the separator positioned between the respective phases. The centrifuge then is stopped, and the elastomeric portion of the separator resiliently returns to its initial shape in sealing engagement with inner surfaces of the tube. Thus, the separator substantially separates the phases of blood and enables the respective phases to be separately analyzed.